Nanoscale structures, such as nanoparticles, nanorods, nanowires, nanocubes, and nanotubes, have attracted extensive synthetic attention as a result of their novel size-dependent properties (Xia et al., Adv. Mater., 2003, 15, 353; Patzke et al., Angew. Chem. Int. Ed., 2002, 41, 2446; Wu et al., Chem. Eur. J., 2002, 8, 1260; Tremel, W., Angew. Chem. Int. Ed., 1999, 38, 2175; Tenne, R., Chem. Eur. J., 2002, 8, 5296; Rao et al., J. Mater. Chem., 2001, 11, 2887; and Rao et al., J. Chem. Soc., Dalton Trans., 2003, (1), 1). In particular, one-dimensional (1-D) materials (e.g., nanorods) with their inherent anisotropy are the smallest dimension structures that can be used for efficient transport of electrons and optical excitations. As such, they are applicable as building blocks to assemble the next generation of molecular electronic and computational devices.
Part of the challenge of developing practical nanoscale devices for a variety of applications, including energy storage, fuel cells, and sensing, is the ability to conveniently synthesize well-characterizable, single-crystalline nanostructures in order to rationally exploit their nanoscale optical, electronic, thermal, and mechanical properties (Rao et al., J. Mater. Chem., 2001, 11, 2887; Duan et al., Nature, 2001, 409, 66; Rueckes et al., Science, 2000, 289, 94; and Gao et al., Nature, 2002, 415, 599).
Ideally, the net result of nanoscale synthesis is the production of structures that achieve monodispersity, stability, and crystallinity with a predictable morphology. Many of the synthetic methods used to attain these goals have been based on principles derived from semiconductor technology, solid state chemistry, and molecular inorganic cluster chemistry.
Strategies for the preparation of 1-D nanowires, for example, rely on the formation from a confined alloy droplet, as described by the vapor-liquid-solid (VLS) mechanism (Wu et al., Chem. Eur. J., 2002, 8, 1260 and Hu et al., Acc. Chem. Res., 1999, 32, 435), the kinetic control of growth through the use of capping reagents (Peng et al., J. Am. Chem. Soc., 2002, 124, 3343 and Puntes et al., Science, 2001, 291, 2115), the generation through a low temperature, chimie douce solution chemical methodology Limmer et al., Adv. Mater., 2001, 13, 1269 and Ginzburg-Margau et al., Chem. Commun., 2002, (24), 3022).
Metal oxides and metal fluorides, in particular, represent two of the most diverse classes of materials, with important structure-related properties, including superconductivity, ferroelectricity, magnetism, conductivity, and gas sensing capabilities (Smart et al. Solid State Chemistry; 2nd ed.; Chapman & Hall: New York, 1995 and West, A. R. Basic Solid State Chemistry; 2nd ed.; John Wiley & Sons: New York, 1999).
Prior art methods of synthesizing oxide nanostructures are by heating and calcination of precursors (Rao et al., J. Chem. Soc., Dalton Trans., 2003, (1), 1); reversed micelle templating techniques (Qi et al., J. Phys. Chem. B, 1997, 101, 3460 and Kwan et al., Chem. Commun., 2001, (5), 447); sol-gel processes (Krumeich et al., J. Am. Chem. Soc., 1999, 121, 8324); surfactant-mediated steps (Wang et al., Chem. Commun., 2001, (8), 727); and hydrothermal procedures (Liao et al., Chem. Mater., 2000, 12, 2819).
Prior art methods of synthesizing fluoride nanostructures are by hydrothermal/solvothermal method or reverse micelle method (Sun et al., Chem. Commun. 2003, 1768; Cao et al., J. Am. Chem. Soc. 2003, 125, 11196; Huang et al., Mater. Lett. 2005, 59, 430 and Agnoli et al., Adv. Mater. 2001, 13, 1697).
However, these prior art methods have substantial shortcomings. Most significantly, the prior art methods do not allow for the synthesis of pure, single-crystalline nanostructures with predictable size and morphology.
For example, although a few prior art methods allow for the synthesis of nanorods with a high aspect ratio, these methods do not yield pure nanostructures. In particular, these methods use organic surfactants in their processes for making nanostructures. (Shi et al., Adv. Mater., 2003, 15, 1647 and Shi et al., Chem. Commun., 2002, (16), 1704.) Thus, the resultant nanostructures have organic surfactant molecular groups, such as bis(2-ethylhexyl)sulphosuccinate, undecylic acid, decylamine, or double-hydrophilic block copolymers, present on their surfaces.
Additionally, the prior art methods do not enable the synthesis of single-crystalline nanostructures. For example, in spite of the variety of different deposition strategies used in the prior art, including electrochemical deposition, electroless deposition, polymerization, sol-gel deposition, and layer-by-layer deposition in nanoporous templates, the resultant nanostructures are polycrystalline. The reason for the observed polycrystallinity is that these prior methods require additional annealing steps at high temperature. (Lakshmi et al., Chem. Mater., 1997, 9, 857; Limmer et al., Adv. Funct. Mater., 2002, 12, 59; Schmid, G., J. Mater. Chem., 2002, 12, 1231; and Hulteen et al., J. Mater. Chem., 1997, 7, 1075.)
Moreover, the prior art methods do not enable the reproducible fabrication of ordered, monodisperse 3-D arrays of 1-D nanomaterials. Such fabrication is critical because assembly of nanoscale components is a key for building functional devices, important for applications including nanoscale electronics and molecular sensing (Colfen et al., Angew. Chem. Int. Ed., 2003, 42, 2350). Specifically, the fabrication of 3-D arrays of nanorods would be useful for optoelectronic applications, such as room-temperature ultraviolet lasing (Huang et al., Science, 2001, 292, 1897). Though a number of preparative methods have been reported for generating these types of nanoscale architectures, none of these techniques appears to work for metal oxides and fluorides (Yang, P., Nature, 2003, 425, 243; Feng et al., J. Am. Chem. Soc., 2004, 126, 62; Tian et al., Nat. Mater., 2003, 2, 821; and Tian et al., J. Am. Chem. Soc., 2003, 125, 12384).
Furthermore, the prior art methods require complex fabrication processes and high temperatures to yield nanostructures.
Accordingly, there remains a need for a low temperature and simplistic method of synthesizing pure single-crystalline nanostructures which allows for controlling the size, in particular the aspect ratio; the extent of monodispersity; and morphology of the resultant nanostructures.